1. Technical Field
The present invention relates to an external additive for toner, toner covered with the external additive, a development agent using the toner, a toner container, and an image forming apparatus.
2. Background Art
Typically, toner for developing latent electrostatic images contains external additives of particulates having an average primary particle diameter of from several nm to several tens nm. For example, hydrophobized silica particulates are used to impart chargeability, fluidity, and hydrophobicity to toner. To sustain chargeability and suppress the variation of charge size in a high temperature and humidity environment, hydrophobized titanium oxide, etc. is used in general. Recently, particles having a large particle diameter such as large silica have begun to be in use as external additives.
By adding such hydrophopbized silica or metal oxides to toner as external additives, the toner can demonstrate fluidity, chargeability, environment stability, etc., which just mother toner particles cannot secure.
Also, as technologies to cover toner particles with external additives, for example, JP-2001-066820-A discloses a particular mono-dispersed spherical silica having a true specific gravity of from 1.3 to 1.9 and a volume average particle diameter of from 80 nm to 300 nm as an external additive to toner. According to JIS 2001-066820-A mentioned above, this particular silica secures fluidity, chargeability, developability, transferability, and fixability of toner at the same time for a long period of time.
In addition, JP-2007-248911-A discloses toner that contains coloring agent particles, external additives, and organic particles having fine pores on its surface which has, for example, a cross-linking density of from 3% by weight to 15% by weight, a total volume of the fine pores of from 0.01 cc/g to 0.50 cc/g, a specific surface area of from 5 m2/g to 50 m2/g, and an average pore diameter of the fine pores of from 0.01 μm to 2.0 μm. According to JP-2007-248911-A mentioned above, toner is provided which can prolong the working life of an image forming apparatus and a development agent in addition to suppressing the degradation of images and also, a development agent, an image forming method, and a process cartridge that use the toner are provided.
In addition, JP4668778-B (JP-2007-156099-A) discloses using at least three kinds of hydrophobic fine powder having different average primary particle diameters as an external additive of toner. According to JP4668778-B (JP-2007-156099-A) mentioned above, toner is provided which sustains good transferability and cleanability for an extended period of time, prevents occurrence of filming on an image bearing member (photoreceptor), suppresses variation of unevenness of images, and in addition, exhibits excellent stability free or little from sinkage of external additives in toner caused by stirring a development agent during usage and with no or little variation of fluidity and chargeability for an extended period of time. Also, there are provided a development agent, a toner container, a process cartridge, and an image forming method that use the toner.
Although methods or technologies including covering the surface of toner particles with surface-treated particulates are successful to some degree, these need further improvement.
For example, problems such as sinkage of external additives present on the surface of mother toner particles, attachment status, prevention of degradation of properties, and detachment. should be solved.
Therefore, early provision of toner having excellent cleanability, image quality, and durability by covering the surface of the toner efficiently with a small amount of additives is in demand. Also, a development agent and an image forming method that use the toner are also in demand.
Conventionally, surface-reforming methods including attaching particulates to the surface of mother toner particles have been employed to impart demanded properties. However, this kind of reforming by external additives has disadvantages such that attached particles are easily detached from mother toner particles or sunk therein due to external stresses. As a result, the surface properties of the toner particles changes, which leads to change of the properties of toner, thereby degrading the high temperature stability and the fixability and changing the charging size, the fluidity, and the agglomeration degree of the toner. As a consequence, the image quality tends to deteriorate, for example, transfer of images becomes poor and back ground fouling occurs.